1. Field of the Invention
The present invention relates to an ion-selective electrode for use in electrochemical measurement of the amount of ions present in a sample solution, and more particularly, to a structure of an internal electrode in the ion-selective electrode having an ion-sensing membrane in a complicated configuration.
2. Description of the Related Art
There have been two types of ion-selective electrode having an ion-sensing membrane: the first type ion-selective electrode has an internal electrolyte solution or internal electrolyte gel on the side opposite to the ion-sensing membrane, which will contact with a sample solution, and is capable of making an electrochemical contact with an internal electrode consisting of silver/silver chloride and the like via the internal electrolyte solution or the internal electrolyte gel; and the second type ion-selective electrode has no internal electrolyte solution, nor internal electrolyte gel on the side opposite to the ion-sensing membrane which will contact with a sample solution, but is capable of making an electrochemical contact directly with the internal electrode consisting of silver/silver chloride and the like. Comparing with the first type ion-selective electrode, the second type one is characterized in that it has a simpler structure and can be easily compacted because of having no internal electrolyte solution. Many ion-selective electrodes of the second type have been known, as disclosed in, for example, JP 58-86449A where a flow-through cell type ion-selective electrode was produced by forming an ion-sensing membrane on the internal surfaces of a cylindrical internal electrode; JP 63-37251A where an ion-selective electrode of the flow-through cell type having a similar structure is disclosed; JP 61-180133A where an attempt is made to enhance the stability and the measurement accuracy of the flow-through cell type ion-selective electrode by providing an ion-sensing membrane projecting convexly into the flow path.
When the ion-sensing membrane is disposed projecting convexly into the flow path of a sample solution, the projection formed to have a smoothly curved surface can facilitate exchanges of samples at the ion-sensing membrane while preventing the flow of the sample solutions from stagnating, which is effective to speed up the response of the ion-selective electrode. However, there is still a problem that it is not easy to make the ion-sensing membrane in a complex three-dimensional form, for example, in a projecing form having a smoothly curved surface. In one of simplest techniques to solve the problem, the flow-through cell may be made of a resin such as poly(vinyl chloride) and shaven along a curved surface projecing into the flow path. An ion-sensing membrane may be separately produced and adhered on the opposite side of the shaven curved surface to the flow path making use of the membrane's flexibility. Then a plate-like internal electrode is adhered on the back side of the ion-sensing membrane to produce an ion-selective electrode. However, the internal electrode made of a metal plate having a sufficient mechanical strength is less flexible so that a gap is apt to be formed between the ion-sensing membrane and the internal electrode unless the configuration of the internal electrode is precisely consistent with the shaven curved surface. Once the gap is formed, a failure in adhesion may be caused during the production of the ion-selective electrode, or the repetition of the use of the ion-selective electrode may render the ion-sensing membrane liable to be peeled off. Therefore, there remains a problem to be solved. An attempt has been made to overcome this problem by making the surface area of a disk-like internal electrode smaller and coating the circular periphery of the disk-like internal electrode with the same polymeric material as the matrix of the ion-sensing membrane so that any gap which might be formed during adhering the internal electrode to the ion-sensing membrane is made smaller to achieve a better adhesion between the internal electrode and the ion-sensing membrane and to ensure good adhering conditions, thereby preventing the adhered regions from delaminating. This technique produces problems that the good gapless adhesion between the internal electrode and the ion-sensing membrane can be achieved at the cost of the effective contacting surface area of the internal electrode and that when an ion-sensing membrane having a high impedance is used, the resultant ion-selective electrode is susceptible to noise generation.